Characterization of the gene encoding herpesvirus of turkeys gp57-65: comparison to Marek's disease virus gp57-65 and herpes simplex virus glycoprotein C

The genome of herpesvirus of turkeys: comparative analysis with Marek's disease viruses

The complete coding sequence of the herpesvirus of turkeys (HVT) unique long (U(L)) region along with the internal repeat regions has been determined. This allows completion of the HVT nucleotide sequence by linkage to the sequence of the unique short (U(S)) region. The genome is approximately 160 kbp and shows extensive similarity in organization to the genomes of Marek's disease virus serotypes 1 and 2 (MDV-1, MDV-2) and other alphaherpesviruses. The HVT genome contains 75 ORFs, with three ORFs present in two copies. Sixty-seven ORFs were identified readily as homologues of other alphaherpesvirus genes. Seven of the remaining eight ORFs are homologous to genes in MDV, but are absent from other herpesviruses. These include a gene with similarity to cellular lipases. The final, HVT-unique gene is a virus homologue of the cellular NR-13 gene, the product of which belongs to the Bcl family of proteins that regulate apoptosis. No other herpesvirus sequenced to date contains a homologue of this gene. Of potential significance is the absence of a complete block of genes within the HVT internal repeat that is present in MDV-1. These include the pp38 and meq genes, which have been implicated in MDV-1-induced T-cell lymphoma. By implication, other genes present in this region of MDV-1, but missing in HVT, may play important roles in the different biological properties of the viruses.

Nucleotide sequence of the gene encoding the major capsid protein of herpesvirus of turkeys

The gene encoding the major capsid protein (MCP) VP5 of herpesvirus of turkeys (HVT) was identified and sequenced. It has a single open reading frame of 4236 nucleotides encoding 1412 aa protein. The gene is flanked by VP23 and UL20 sequences and is localized in the unique long region (UL) within the BamHI-B fragment. Comparison of amino acid homology has shown its clear position among the alpha-herpesviruses rather than beta- or gamma-herpesviruses. The VP5 is expressed from polycistronic mRNA together with the UL20 and the VP23 genes. The 7,2 kb RNA transcript is lacking any promoter elements or polyA signal in intergenomic regions between VP5 and UL20 or VP5 and VP23 genes, respectively. Multiple alignment of known major capsid protein sequences of all herpesvirus groups revealed presence of seven highly homologous clusters suggesting-that the corresponding protein domains might play an important role in folding of MCP and assembly of herpesvirus capsid.

Open reading frames in a 4556 nucleotide sequence within MDV-1 BamHI-D DNA fragment: evidence for splicing of mRNA from a new viral glycoprotein gene

A DNA segment of the MDV-1 BamHI-D fragment was sequenced, and the open reading frames (ORFs) present in the 4556 nucleotide fragment were analyzed by computer programs. Computer analysis identified 19 putative ORFs in the sequence ranging from a coding capacity of 37 amino acids (aa) (ORF-1a) to 684aa (ORF-1). The special properties of four ORFs (1a, 1, 2, and 3) were investigated. Two adjacent ORFs, ORF-1a and ORF-1, were found by computer analysis to have the properties of two introns encoding a glycoprotein: ORF-1a encodes an aa sequence with the properties of a signal peptide, and ORF-1 encodes a polypeptide with a membrane anchor domain and putative N-glycosylation sites in the aa sequence. ORF-1a and ORF-1 were found to be transcribed in MDV-1-infected cells. Two RNA transcripts were detected: a precursor RNA and its spliced form. Both are transcribed from a promoter located 5' to ORF-1a, and splice donor and acceptor sites are used to splice the mRNA after cleavage of a 71-nucleotide sequence. This finding suggest that ORF-1a and ORF-1 are two introns of a new MDV-1 glycoprotein gene. The DNA sequence containing ORF-1 was transiently expressed in COS-1 cells, and the viral protein produced in these cells was found to react with anti-MDV serotype-1 Antigen B-specific monoclonal antibodies. These studies indicate that the protein encoded by ORF-1 has antigenic properties resembling Antigen B of MDV-1. A gene homologous to ORF-1 was detected in the genome of both MDV-2(SB1) and MDV-3(HVT), which serve as commercial vaccine strains. Two additional ORFs were noted in the 4556 nucleotide sequence: ORF-2, which encodes a 333 aa polypeptide initiating in the UL and terminating in the TRL prior to the putative origin of replication, and ORF-3, which encodes a 155 aa polypeptide that is partly homologous to the phosphoprotein pp38 encoded by the BamHI-H sequence. The 65 N-terminal aa of the two gene products are identical, both being derived from the nucleotide sequences in the TRL and IRL, respectively. Additional homologous aa sequences are the hydrophobic aa domain in the middle of both proteins. The functions of ORF-2, ORF-3, and additional ORFs are under study.

Expression of the Marek's disease virus homolog of herpes simplex virus glycoprotein B in Escherichia coli and its identification as B antigen

Marek's disease (MD) is an oncogenic disease of chickens caused by MD virus (MDV). Among the major glycoproteins found in MDV-infected cells are gp100, gp60, and gp49, detected by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis with antisera previously shown to be reactive with B antigen in immunodiffusion analysis. Following treatment with tunicamycin (TM), an inhibitor of N-linked glycosylation, the same sera were reported to detect two molecules called pr88 and pr44. However, the gene encoding B antigen was not unequivocally identified. Recently, an MDV homolog of the gene encoding herpes simplex virus glycoprotein B (gB) was identified and sequenced (L. J. N. Ross, M. Sanderson, S. D. Scott, M. M. Binns, T. Doel, and B. Milne, J. Gen. Virol. 70:1789-1804, 1989). To determine whether the MDV gB homolog gene might encode the B antigen, antisera against trpE fusion proteins of the MDV gB homolog (trpE-MDV-gB) were prepared. These antisera immunoprecipitated gp100, gp60, gp49, and a 92-kDa precursor polypeptide (pr88, now designated 92-kDa pr88, in the presence of TM) from MDV-infected cell lysates. On the basis of size comparison, trpE-MDV-gB competition and blocking assays, and the fact that gp100, gp60, gp49, and 92-kDa pr88 could be detected in MDV-infected cells with antisera specific to both MDV B antigen and the gB homolog, it was concluded that (i) the MDV gB homolog gene encodes MDV B antigen and (ii) 92-kDa pr88 is the primary precursor polypeptide. The antisera against trpE-MDV-gB also contained antibody reactive with the herpesvirus of turkey gB homolog, consistent with the known antigenic relatedness between the MDV and herpesvirus of turkey B antigens. TM inhibition data and results from pulse-chase analysis with MDV-infected cells show that MDV gB homolog processing involves cotranslational glycosylation of 92-kDa pr88 to form gp100, which is then cleaved to form gp60 and gp49, the N- and C-terminal halves, respectively, of gp100. This processing pathway is consistent with those of other gB homologs, further supporting the gene identification described above. The conclusions of this study will facilitate future research on the immunobiology of MD, especially studies on the mechanism of immunoprotection.

Monospecific antibodies to Marek's disease virus antigen B dimer (200 kDa) and monomer (130 and 60 kDa) glycoproteins neutralize virus infectivity and detect the antigen B proteins in infected cell membranes

Monospecific antibodies were prepared by nitrocellulose blot immunoaffinity to 3 polypeptide components of the host-membrane associated B antigen of Marek's disease herpesvirus (MDV) and to its soluble A antigen. The B antigen comprised a 200 kDa dimer which is 2-mercaptoethanol (2-ME) labile, a monomer of 130 kDa and a 60 kDa protein, both of which are 2-ME resistant. Cross-immunoblotting studies showed that the anti-dimer antibody recognized the dimer protein as well as the 130 and 60 kDa components. In contrast, the anti-130 kDa antibody gave the strongest signal on blots of reducing gels indicating that the monomer is largely formed by in vitro reduction with 2-ME. All four antibodies recognized membrane antigens on chicken embryo fibroblasts infected with MDV vaccine viruses representative of the three serotypes and in addition, neutralized the homologous MDV isolate. The anti-dimer antibody was greatest, the anti-monomer antibody was the weakest and the anti-60 kDa antibody intermediate in neutralizing efficacy to all four viruses. We conclude from these studies that the B antigen presents at least two classes of neutralizing epitopes: one is discontinuous and of broad specificity on the intact dimer molecule and the other, on the 130 and 60 kDa proteins, is continuous and of lower avidity.

Transactivation of the Rous sarcoma virus long terminal repeat promoter by Marek's disease virus

Transient expression of chloramphenicol acetyltransferase (CAT) was used to study Marek's diseases virus (MDV)-mediated transactivation of the Rous sarcoma virus long terminal repeat (RSV-LTR) promoter. Cotransfection experiments in primary avian cells were conducted using MDV high-molecular-weight DNA and plasmid pRSVcat. Increased CAT activity, relative to controls, was consistently observed in the presence of MDV. Enhanced CAT activity, expressed via the RSV-LTR promoter, was strictly dependent on the presence of MDV DNA or virus, suggesting that activation of the RSV-LTR promoter was due to factors expressed in MDV-infected cells. Differences in transactivation efficiency were observed between various strains and the serotypes of MDV. In particular, high- and low-passage pairs of serotype 1 MDV showed marked differences in their ability to increase CAT activity in pRSVcat-transfected cells. Attenuation of viral pathogenicity and decreased expression of some cell surface glycoproteins occur in high-passage MDV strains. Decreased transactivation ability in these same strains suggests that continuous passage in culture and attenuation may perturb a regulatory mechanism operating by transcriptional control. In addition, transactivation of the RSV-LTR promoter suggests that increased incidence of avian leukosis following vaccination by MDV may be due to MDV-mediated transactivation of endogenous ALV proviral LTR promoters. MDV-mediated transactivation was not limited to the RSV-LTR promoter. Serotype 3 MDV (HVT) efficiently transactivated the herpes simplex virus (HSV) alpha 4 (ICP4) and beta-TK promoters as well as the human cytomegalovirus (hCMV) immediate early promoter.